The present invention generally relates to power system protection, and more specifically, to an apparatus and method for compensating secondary currents used in differential protection to correct for a phase shift introduced between high voltage and low voltage transformer windings.
Electric utility systems or power systems are designed to generate, transmit and distribute electrical energy to loads. In order to accomplish this, power systems generally include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, buses and capacitors, to name a few. As a result, power systems must also include protective devices and procedures to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like.
Protective devices and procedures act to isolate some power system element(s) from the remainder of the power system upon detection of the abnormal condition or a fault in, or related to, the power system element(s). Logically grouped zones of protection, or protection zones utilizing the protective devices and procedures, are established to efficiently manage faults or other abnormal conditions occurring in the power system elements.
In general, protection zones may be classified into six categories based on the type of power system elements to be protected. The categories include: (1) generators and generator-transformer elements (2) transformers, (3) buses, (4) lines (transmission, sub-transmission and distribution), (5) utilization equipment (motors, static loads), and (6) capacitor or reactor banks. As a result, a variety of protective devices are required. Such protective devices may include different types of protective relays, surge protectors, arc gaps and associated circuit breakers and reclosures.
Although the fundaments of power system protection are similar, each of the six categories of protection zones use protective devices that are based on the characteristics of the power system elements in that category. More specifically, a variety of types of protective relays utilizing a number of protective schemes (e.g., differential current comparisons, magnitude comparisons, frequency sensing), are required to protect the various power system elements. For example, a current differential relay, having nn electrical connections, is designed to monitor current flowing into a power system element (e.g., a power transformer) by measuring the current flowing into the power system element and calculating inter alia, the sum of all measured current, or the operate current. As is known, when operating under normal conditions, the sum of all of the (primary) currents entering the power system element is about zero (Kirchhoff's current law). If the power system element has a short circuit, or is faulted, its operate current will be substantially different from zero, indicating that there is some impermissible path through which a current is flowing. If the operate current exceeds some threshold, or pickup current, the current differential relay issues a tripping signal to an associated power circuit breaker(s), causing it to open and isolate the faulted power system element from the remainder of the power system.
Because currents resulting from a fault can easily exceed 10,000 amperes (amps) and because a protective device, such as the current differential relay described above, is designed to measure currents up to 100 amps via its nn electrical connections, the protective device is coupled to the power system element(s) via a number of current transformers. The current transformers operate to proportionally step-down the power system current (while retaining the same phase relation) flowing into the protected power system element, to a magnitude that can be readily monitored and measured by the protective device. As is known, the three-phase current flowing into the protected element is commonly referred to as a primary current, and the current flowing from the current transformers to the protective device is commonly referred to as a secondary current. The resulting lower secondary currents are used by the protective device to determine corresponding phasors representative of the primary current. The phasors are then used in the various overcurrent, directional, distance, differential, and frequency protective logic schemes of the various protective devices.
Because of potential relay mis-operation, current differential relays are typically designed with a restraint mechanism intended to restrain the current differential relay under certain circumstances (e.g., prevent it from issuing an erroneous trip signal). One restraint mechanism includes increasing the pickup current of the current differential relay as the currents entering the protected element increase. For example, Equation (1) illustrates one example of calculating the operate current for a current differential relay that utilizes a restraint mechanism.Ioperate>Ipickup+k·Irestraint  (1)where Ioperate=|Ī1+Ī2+Ī3+ . . . Īn, and I restraint=Ī1+Ī2+Ī3+ . . . Īn|, andk=constant. Using one differential scheme, when the operate current Ioperate exceeds the sum of the pickup threshold current Ipickup plus the product of some constant and the sum of the magnitudes of all the currents k·Irestraint entering the protected element, a fault is declared for the protected power system element and the current differential relay issues a tripping signal. The equations described above may be easily modified to accommodate a typical three-phase power system. For example, IA—operate is the operate current of an A-phase differential element of the current differential relay and IA1 represents the secondary current from the A-phase current transformer, IB—operate is the operate current of a B-phase differential element of the current differential relay and IB1 represents the secondary current from the B-phase current transformer, and Ic—operate is the operate current of a C-phase differential element of the current differential relay and IC1 represents the secondary current from the C-phase current transformer. Alternate differential schemes may also be used when comparing the operate and restraint currents.
Current differential relays are commonly used to protect power transformers having a first high voltage (HV) winding and a second low voltage (LV) winding. For ease of discussion, such current differential relays are referred to herein as power transformer differential relays that are configured to monitor the three-phase current on both the HV busbar side and the LV busbar side of the power transformer, via secondary currents provided by respective current transformer groups.
As is known, the HV and the LV windings of a power transformer may be arranged using one of a number arrangements (or combinations of the arrangements) such as a wye-wye configuration, a delta-delta configuration, a wye-delta configuration and a delta-wye configuration, to name a few. For like-windings arrangements such as the wye-wye arrangement, there is normally no angular displacement, or phase shift, between the HV and LV windings. For all other winding arrangements, there is an angular displacement of 30 degrees (or multiples thereof) between the currents of the HV and LV windings.
Prior to becoming operation in the power system, the power transformer and its associated power transformer differential relay are tested under no-load conditions, or “commissioned”. Such commissioning involves many factors, and generally includes checking for errors associated with current transformer (CT) installation. For example, commissioning using traditional electromechanical relays required labor intensive manual intervention by a commissioning engineer to check for errors such as improper current transformer installation which may result in incorrect CT polarities. Other manual tests required during commissioning included ensuring that the current transformer group, or the three individual CTs, at a particular voltage level (e.g., the HV busbar) have the same current transformer ratios, or the same CT tapping.
Various systems and methods have been proposed to minimize manual intervention during relay commissioning. For example, U.S. Pat. No. 5,276,402, entitled “Three-phase Transformer Testing Method and System”, issued Jan. 4, 1994 to inventor Schucht, proposed a three-phase transformer testing method and system that enabled automatic measurement of core loss, load loss and transformer ratio, and performance of polarity checks and phase-relationships. In another example, U.S. Pat. No. 4,758,774, entitled “Portable Tester and Related Method for Determining the Primary Winding to Secondary Winding Current Ratio of an In-service Current Transformer”, issued Jul. 19, 1988 to inventors Crawford et al, described a portable tester for determining the primary winding to secondary winding current ratio of a current transformer while the transformer is coupled to the power system.
While requiring some manual intervention, the microprocessor-based, or numerical, relays can be programmed to assist the commissioning engineer in the commissioning process. Accordingly, a numerical power transformer differential relay can be programmed to automatically determine (1) whether an incorrect CT polarity is present, (2) whether one or more crossed phases is present (e.g., improper current transformer wiring causing phase-B secondary current to be received by the protective relay as phase-C current element, and vice versa) and (3) whether an incorrect CT ratio exists (e.g., an incorrect TAP connection of the A-phase transformer). For example, Young and Horak, in their paper entitled “Commissioning Numerical Relays”, described a system that tests for CT wiring errors such as incorrect CT polarities, incorrect CT ratios and cross-phased wiring errors. Unfortunately, none of the prior art systems and methods for commissioning provided a way to automatically compensate secondary currents used by current differential protection schemes to correct for a phase shift occurring between the currents of the HV winding and the LV winding of a power transformer. When not corrected, such a phase shift between the currents of the HV winding and the LV winding may result in power transformer differential relay mis-operation.